Systems neuroscience aims to explain animal behavior via a thorough understanding of nervous system organization and function. In order to reach a comprehensive understanding, one must utilize both classical neurobiological and biophysical approaches to identify neuronal substrates for behavior while capturing the intrinsically computational nature of these circuits. The nervous systems of high order mammals are so impressively complex (100 billion neurons in the human brain), that they cannot be understood at this scale. For this reason, one can instead study behavior in facile genetic model organisms, such as the Drosophila larva, to rapidly gain a systems level understanding of behavioral traits and reveal conserved principles of neuroscience that apply to all animals. The Drosophila larva senses light via their Bolwig's organs, eye-like structures on the surface of the larval head. Larvae exhibit a strong navigational preference for darkness, but little is known about how the larva utilizes photosensory information to systematically pattern its own crawling movements to explore and seek preferred regions within photosensory landscapes. To do this, I developed a tracking system to quantify the detailed movements of individual larvae as they respond to defined spatiotemporal patterns of luminosity. In regions of constant luminosity, a larva's trajectory is a sequence of runs (periods of forward movement) that are interrupted at random by abrupt turning events, during which the larva pauses and sweeps its head back and forth until it starts a new run in a new direction. I find that larvae modulate their turn direction, turn probability and turn size in response to photosensory input to yield trajectories in which most of its time is in the dark. These rules for navigating luminosity gradients provide insight into the underlying sensorimotor transformations that are carried out by neural circuits for phototaxis. In this proposal, I outline experiments to identify these circuits by (1) further defining the sensory input and behavioral parameters that underlie phototaxis navigational strategy and (2) determine how photosensory information is encoded in the nervous system at the level of photoreceptors and downstream central brain circuits. PUBLIC HEALTH RELEVANCE: In order to understand nervous system function in higher order animals, one must be able to identify both the neuronal site and computational task of a behavior generating circuit. Attaining this level of resolution for the phototaxis circuit will inform dissection of behavior generating circuits in higher animals, and provide insight into their proper function and malfunction in disease states.